This invention relates to the making of weathering high strength thin cast strip, and to the method for making such cast strip by a twin roll caster.
Weathering steel is a high strength low alloy steel resistant to atmospheric corrosion. In the presence of moisture and air, low alloy steels oxidize, the rate of which depends on the access of oxygen, moisture and atmospheric contaminants to the metal surface. As the process progresses, the oxide layer forms a barrier to the ingress of oxygen, moisture and contaminants, and the rate of rusting slows down. With weathering steel, the oxidation process is initiated in the same way, but the specific alloying elements in the steel produce a stable protective oxide layer that adheres to the base metal, and is much less porous. The result is a much lower corrosion rate than would be found on ordinary structural steel.
Weathering steels are defined in ASTM A606, Standard Specification for Steel, Sheet and Strip, High Strength, Low-Alloy, Hot Rolled and Cold Rolled with Improved Atmospheric Corrosion Resistance. Weathering steels are supplied in two types: Type 2, which contains at least 0.20% copper based on cast or heat analysis (0.18% minimum Cu for product check); and Type 4, which contains additional alloying elements to provide a corrosion index of at least 6.0 as calculated by ASTM G101, Standard Guide for Estimating the Atmospheric Corrosion Resistance of Low-Alloy Steels, and provides a level of corrosion resistance substantially better than that of carbon steels with or without copper addition.
Weathering steel's yield strength allows cost reduction through the ability to design lighter sections into structures. In the past, high strength weathering low-carbon thin strip has been made by recovery annealing of cold rolled strip. Cold rolling was required to produce the desired thickness. The cold rolled strip was then recovery annealed to improve ductility without significantly reducing the strength. However, the final ductility of the resulting strip still was relatively low and the strip would not achieve total elongation levels over 6%, which is required for structural steels by building codes. Such recovery annealed cold rolled, low-carbon steel was generally suitable only for simple forming operations, e.g., roll forming and bending. To produce this steel strip with higher ductility was not technically feasible in these final strip thicknesses using the cold rolled and recovery annealed manufacturing route.
High strength weathering low-carbon steel strip has also been made by microalloying with elements such as niobium (Nb), vanadium (V), titanium (Ti) or molybdenum (Mo), and hot rolling to achieve the desired thickness and strength level. Additions of nickel (Ni), copper (Cu) and silicon (Si) to the microalloying were used to obtain the corrosion-resistance properties. Microalloying required expensive and high levels of niobium, vanadium, titanium or molybdenum and resulted in formation of a bainite-ferrite microstructure typically with 10% to 20% bainite. Alternately, the microalloying could result in formation of a ferrite microstructure with 10% to 20% pearlite.
Hot rolling the strip resulted in the partial precipitation of these alloying elements. As a result, relatively high alloying levels of the Nb, V, Ti or Mo elements were required to provide enough precipitation hardening of the predominately ferritic transformed microstructure to achieve the required strength levels. These high microalloying levels significantly raised the hot rolling loads needed and restricted the thickness range of the hot rolled strip that could be economically and practically produced.
As such, making of high strength low-carbon steel strip less than 4 mm in thickness with microalloying additions of Nb, V, Ti and/or Mo to the base steel chemistry has been very difficult, particularly for wide strip due to the high rolling loads, and not always commercially feasible. For thinner thicknesses of strip, cold rolling was required; however, the high strength of the hot rolled strip made such cold rolling difficult because of the high cold roll loadings required to reduce the thickness of the strip. These high alloying levels also considerably raised the recrystallization annealing temperature needed, requiring expensive to build and difficult to operate annealing lines capable of achieving the high annealing temperature needed for full recrystallization annealing of the cold rolled strip.
Addition of phosphorus is also currently used to improve machining characteristics and atmospheric corrosion resistance in steels. For example, Chinese Patent Application Publications Nos. CN103305759, CN103302255, and CN103305770, all show purposeful addition of phosphorus between 0.07% to 0.22% to improve corrosion resistance of steel composition. However, phosphorus causes embrittlement which reduces toughness and ductility. For example, phosphorus causes temper embrittlement in heat-treated low-alloy steels resulting from segregation of phosphorus and other impurities at prior austenite grain boundaries. Additionally, phosphorus content greater than 0.04% makes weld brittle and increases the tendency to crack. The surface tension of the molten weld metal is lowered, making it difficult to control.
In short, the application of previously known microalloying practices with Ni, V, Ti and/or Mo elements and the purposeful addition of phosphorus to produce high strength weathering low-carbon thin strip are not practicable methods. The high alloying costs, difficulties with high rolling loads in hot rolling and cold rolling, the high recrystallization annealing temperatures required, and phosphorus harmful effects are problems with the existing process for manufacturing high strength weathering steel. As such, there is still a need for developing an economically feasible and effective method to produce high strength weathering or corrosion-resistant thin steel.
Disclosed is a method of making weathering steel comprising the steps of: preparing a molten melt producing an as-cast carbon alloy steel strip less or equal to 4 mm in thickness with a corrosion index of at least 6.0 comprising, by weight, between 0.02% and 0.08% carbon, less than 0.6% silicon, between 0.2% and 2.0% manganese, less than 0.03% phosphorus, less than 0.01% sulfur, less than 0.01% nitrogen, between 0.2% and 0.5% copper, between 0.01% and 0.2% niobium, between 0.01% and 0.2% vanadium, between 0.1% and 0.4% chromium, between 0.08% and 0.25% nickel, less than 0.01% aluminum, and the remainder iron and impurities from making the molten melt; solidifying and cooling the molten melt into a cast strip less than or equal to 4 mm in thickness in a non-oxidizing atmosphere; hot rolling the cast strip in an austenitic temperature range above Ar3 to between 10% and 50% reduction; cooling the hot rolled cast strip at above 20° C. per second; coiling the cast strip below 700° C. to form a steel strip with a microstructure comprising bainite and acicular ferrite with more than 70% niobium in solid solution; and age hardening the steel strip forming an age hardened steel strip having a yield strength of at least 550 MPa and a total elongation of at least 8%.
The age hardened steel strip may be batch annealed at a temperature greater than 450° C. between 15 and 50 hours. The age hardened steel strip by batch annealing may have a yield strength of at least 700 MPa and a total elongation of at least 8%.
Alternatively, the age hardened cast strip may be in-line annealed at a temperature between 450° C. and 800° C. for less than 30 minutes. The age hardened steel strip by in-line annealing may have a yield strength of at least 700 MPa and a total elongation of at least 8%.
Also disclosed is a method of continuously casting weathering steel comprising the steps of: assembling a pair of counter-rotatable casting rolls to form a nip there between through which a thin strip can be casted, and capable of supporting a casting pool of molten metal formed on casting surfaces of the casting rolls above the nip with a pair of confining side dams adjacent the ends of the casting rolls; assembling a delivery system with metal delivery nozzle or nozzles disposed axially above the nip and capable of discharging molten metal to form the casting pool supported on the casting rolls; solidifying the molten metal delivered from the casting pool on the casting surfaces of the casting rolls in a non-oxidizing atmosphere and forming at the nip between the casting rolls a cast strip delivered downwardly that is less than 4 mm in thickness with a corrosion index of at least 6.0 comprising, by weight, of between 0.02% and 0.08% carbon, less than 0.6% silicon, between 0.2% and 2.0% manganese, less than 0.03% phosphorus, less than 0.01% sulfur, less than 0.01% nitrogen, between 0.2% and 0.5% copper, between 0.01% and 0.2% niobium, between 0.01% and 0.2% vanadium, between 0.1% and 0.4% chromium, between 0.08% and 0.25% nickel, less than 0.01% aluminum, and the remainder iron and impurities from melting; hot rolling the cast strip in an austenitic temperature range above Ar3 to between 10% and 50% reduction; cooling the hot rolled cast strip at above 20° C. per second; coiling the cast strip below 700° C. to form a steel strip with a microstructure comprising bainite and acicular ferrite with more than 70% niobium in solid solution; and age hardening the steel strip forming an age hardened steel having a yield strength of at least 550 MPa and a total elongation of at least 8%.
The age hardened steel strip may be batch annealed at a temperature greater than 450° C. between 15 and 50 hours. The age hardened steel strip by batch annealing may have a yield strength of at least 700 MPa and a total elongation of at least 8%.
Alternatively, the age hardened cast strip may be in-line annealed at a temperature between 450° C. and 800° C. for less than 30 minutes. The age hardened steel strip by in-line annealing may have a yield strength of at least 700 MPa and a total elongation of at least 8%.
Also disclosed is a weathering steel made by preparing a molten melt producing an as-cast carbon alloy steel strip less or equal to 4 mm in thickness with a corrosion index of at least 6.0 comprising by weight, between 0.02% and 0.08% carbon, less than 0.6% silicon, between 0.2% and 2.0% manganese, less than 0.03% phosphorus, less than 0.01% sulfur, less than 0.01% nitrogen, between 0.2% and 0.5% copper, between 0.01% and 0.2% niobium, between 0.01% and 0.2% vanadium, between 0.1% and 0.4% chromium, between 0.08% and 0.25% nickel, less than 0.01% aluminum, and the remainder iron and impurities from making the molten melt; solidifying and cooling the molten melt into a cast strip less than or equal to 4 mm in thickness in a non-oxidizing atmosphere; hot rolling the cast strip in an austenitic temperature range above Ar3 to between 10% and 50% reduction; cooling the hot rolled cast strip at above 20° C. per second; coiling the cast strip below 700° C. to form a steel strip with a microstructure comprising bainite and acicular ferrite with more than 70% niobium in solid solution; and age hardening the steel strip forming an age hardened steel strip having a yield strength of at least 550 MPa and a total elongation of at least 8%.
Again, the age hardened steel strip may be batch annealed at a temperature greater than 450° C. between 15 and 50 hours. The age hardened steel strip may have a yield strength of at least 700 MPa and a total elongation of at least 8%. Alternatively, the age hardened cast strip may be in-line annealed at a temperature between 450° C. and 800° C. for less than 30 minutes. The age hardened steel strip may have a yield strength of at least 700 MPa and a total elongation of at least 8%.